Pottu, this is the one that should be stickied in my humble opinion.
I would really like to see if we could get at least some rough numbers to quantify stats, though solid numbers (at least from a mana regeneration standpoint) would be challenging. I'll just write my thoughts and hopefully some conversation can come from it and someone would do some testing that would help us out.
Spell power, hit, and crit are very straigtforward with what they do. I'm not much of an expert at theory of spell damage or how resists work. But lets just talk logically about them. Spell hit reduces the number of times that resists will be cast. You could break down a fight to look at 100 casts and see what average damage would be for a set amount of spell power. Most casts would hit, some would crit, some would resist.
Changing the hit would add more hits and less resists. Those hits would also possibly crit so all of that would need to be averaged but you could get a set damage increase because of the hit. When you have the damage increase you would then be able to compare it to what an x amount of spell power would do under the same circumstances. You could also do the same methodology for crit. All of this you could crunch with proper gear swapping and looking at the theorycraft addon.
Intellect provides spell crit but it also provides more mana pool. For an oomkin that means more spells cast.
Spirit and MP5 provides the opportunity over the course of the fight to cast more spells. Spi gets 15% with talents.
The problem with the extra spells cast with int, spi, and mp5 is that it's not just the number of spells that can be cast extra, but how powerful they are. All of these stats will be more valuable with better basic stats like spell power.
Going through this and just thinking about it I think this actually can be quantified. So a moonkin with theorycraft and some knowledge would need to do some calculations. Theorycraft gives a mana cost to a spell and a real mana cost. The real mana cost takes into account mana that will be regenerated during the course of the cast time. Since regeneration is a matter of percentage of number of casts the scaling with spell, hit, or crit would be linear (I think?) so we could actually put a real value to those stats. I'm sure the value is small but we could at least quantify it.
Quantifying int as just a larger mana pool is tricky though because it is a one time use as opposed to spi or mp5 which is a constantly regenerating number. Best I could think is that we could just take a fully buffed moonkin and see how many casts of a spell you get on average and then do math magic to see how much value getting that extra spell cast or two would have over the course of the fight.